[DAGCombiner] narrow vector binop with 2 insert subvector operands

vecbo (insertsubv undef, X, Z), (insertsubv undef, Y, Z) --> insertsubv VecC, (vecbo X, Y), Z

This is another step in generic vector narrowing. It's also a step towards more horizontal op 
formation specifically for x86 (although we still failed to match those in the affected tests).

The scalarization cases are also not optimal (we should be scalarizing those), but it's still 
an improvement to use a narrower vector op when we know part of the result must be constant 
because both inputs are undef in some vector lanes.

I think a similar match but checking for a constant operand might help some of the cases in 
D51553.

Differential Revision: https://reviews.llvm.org/D56875

llvm-svn: 351825